A. Field of the Invention
The present invention relates to a method for hydrolyzing starch generally which eliminates the limit dextrin problem encountered in methods that employ the alpha-amylase enzyme. This invention further relates to an enzymatic process for the production of high solids dextrin adhesives directly from raw starch. The high solids dextrin adhesives produced by the disclosed method are useful in the high speed machine coating of paper, and also the wrinkle free conversion of paper to various end products such as envelopes and poster board.
Raw starch can be converted into a number of commercial products such as laundry starches, sizing for paper, and a variety of adhesives. Most of these commercial products require that the raw starch by hydrolyzed or cleaved into fragments. It is the size and shape of the fragments so produced which impart the properties that distinguish one starch product from another. This is especially true in the case of adhesives where particle size and shape will determine whether an adhesive is a low solids type suitable for the production of gypsum board or whether the adhesive is a high solids type suited for lay flat mounting or for the high speed machine application of envelope back gums.
Starch (C.sub.6 H.sub.10 O.sub.5)n, is the principal reserve polysaccharide in plants. In plants, starch usually occurs as granules. Starch granules are broken down into useful commercial products, such as adhesives, based upon their physical or chemical properties or both.
One of the physical properties of the starch granule is its ability to gelatinize. Gelatinization is a process wherein hot water acts upon a slurry or aqueous suspension of starch granules to overcome hydrogen bonding within the granule causing the granules to swell. The temperature at which swelling begins to occur is the gelatinization temperature. As the temperature increases, there is an increase in the number and size of the swollen granules and a corresponding increase in the viscosity of the slurry. At first, the viscosity of the slurry increases rapidly. However, because the swollen granules are fragile and tend to fragment under the influence of shear in the cooking vessel, there is a corresponding reduction in viscosity with fragmentation. As a consequence of these competing effects, the viscosity of a heated slurry of starch granules increases rapidly when the gelatinization temperature is reached; peaks when fragmentation equals gelatinization; decreases as fragmentation becomes predominent over swelling, and finally stabilizes when both further swelling and fragmentation becomes negligible. The fragments that are produced by this process are called "solids". In any given reaction slurry, as the amount of fragmentation increases, the solids content of the slurry increases and its viscosity decreases. The production of reaction slurries having a high solids content is highly desirable in the paper adhesives industry.
The chemical properties of starch are based upon its structure. Amylose is the linear polymer of starch and it is composed exclusively of .alpha.-D-glucopyranosyl units linked (1.fwdarw.4). Amylopectin, on the other hand, is the branched polymer of starch which is composed of .alpha.-D-glucopyranosyl units linked (1.fwdarw.4) with (1.fwdarw.6) linkages creating the branch points.
B. Prior Art
The classic method for producing adhesives in general from an aqueous slurry of raw starch takes advantage of the physical properties of the starch granule. The classic method consists of heating the slurry of the raw starch to past its gelatinization point while subjecting the extremely long and highly viscous starch molecules to mechanical shearing action. The function of the shearing action is to break the long starch molecules into shorter and less viscous molecules without destroying the adhesiveness of the native starch molecule. Although this procedure is suited for the production of low solids adhesives, it is not suited to producing the high solids starch adhesives of the present invention. See: U.S. Pat. No. 3,450,549 (Schwalbe) at col. 2, ln. 1-4.
Pigman (U.S. Pat. No. 2,609,326) describes the early use of a single thermally stable enzyme, alpha-amylase, to depolymerize raw corn starch into a dry laundry starch product that is readily soluble in cold water. The enzyme, alpha amylase, acts upon a chemical property of the starch--the .alpha.(1.fwdarw.4) linkage. Specifically, the enzyme, alpha amylase, partially depolymerizes the raw starch by randomly hydrolyzing only the .alpha.(1.fwdarw.4) glucosidic linkages in the gelatinized starch granules.
Schwalbe (U.S. Pat. No. 3,450,549) describes the use of the thermally stable enzyme, alpha amylase, in the production of high solids dextrin adhesives. In Schwalbe, the alpha amylase enzyme is used to first subject the gelatinized native starch "to a regulated degree of hydrolysis", i.e.,--to a pre-determined reduction in viscosity. (Col. 2, ln. 22-24.) According to Schwalbe, complete enzymatic hydrolysis of the starch is undesirable because the hydrolysis process produces dextrinized products (limit dextrins) which materially reduce the bonding action of the starch adhesive. (Col. 2, ln. 53-59). Accordingly, the "regulated degree of enzymatic hydrolysis" described in Schwalbe consists of permitting the enzyme, alpha-amylase, to randomly cleave some linear starch fragments off both the outer branches of amylopectin starch molecules and off the linear amylose molecules. In Schwalbe, the critical step consists of preventing the alpha-amylase enzyme from cleaving all the linear fragments off the outer branches of the amylopectin molecule thereby producing a terminally branched limit dextrin--a large starch molecule whose terminal branches contain nonhydrolyzable .alpha.(1.fwdarw.6) glucosidic linkages. (Col. 2, ln. 53-65).
In Schwalbe, upon achieving the "re gulated degree of enzymatic hydrolysis", the alpha-amylase enzyme is then denatured. Further breakdown of the starch molecules is then accomplished by employing a second step--a conventional mechanical shearing step (steam jetting or high speed agitation) which is conducted at temperatures in the range of 260.degree.-300.degree. F. until the desired reduction in viscosity is obtained. (Col. 3, ln. 11-19).
Black (U.S. Pat. No. 4,014,743) describes a continuous flow enzymatic process for decreasing the viscosity (thinning) of starch slurrys to eliminate problems resulting from the development of temporary peak viscosities during geletinization in the usual enzyme liquification procedures. Black's process consists of continuously adding the enzyme (alpha-amylase) and raw starch slurry to a gelatinized and partially converted reaction slurry so as to maintain a blend within the tank having a low enough viscosity to be agitated and pumped off. In a mechanical shearing step similar to that described in Schwalbe, Black mixes the effluent from the alpha-amylase conversion with steam in a jet mixer-heater at a temperature raised to 320.degree. F. (Col. 7, ln. 44-47), further reducing viscosity. This step has the further function of denaturing the enzyme.